Lorentz TEM study of the magnetic microstructure in near-eutectoid Co-Pt alloys

•The magnetic microstructure of a new-eutectoid Co-Pt alloy was investigated.•An unconventional inter-phase magnetic domain wall was discovered.•Domain walls in tweed and macro-twinned microstructures are reported. In this paper, we report Lorentz Transmission Electron Microscopy (LTEM) observations of magnetic domain structures in a near-eutectoid Co40Pt60 alloy. The crystallographic microstructure is characterized using conventional bright field/dark field TEM imaging. The magnetic induction orientation inside magnetic domains is extracted from Fresnel through-focus images by reconstructing the phase of electron wave using the transport-of-intensity equation. The alloy shows a nano-chessboard pattern composed of the L10 tetragonal and L12 cubic phases. The magnetization distribution in four neighboring L10 tiles in the nano-chessboard structure is found to follow a vortex/anti-vortex configuration to reduce the magnetostatic and magneto-crystalline anisotropy energies. An unconventional domain wall referred to as an inter phase magnetic domain wall (IPMDW) is observed at the inter-phase boundaries of L10 and L12 phases. Magnetic domain walls in other microstructures (tweed microstructure, macro-twinned structure, coarsened L10 plates) are also documented.